The state's abundant harvest of fruits and nuts is threatened by warmer winters, UC Davis scientists warn in a new report.

All fruit and nut trees require periods of dormancy each winter before they can bloom again, and it's the cold that keeps them dormant. When the length of that chill is disrupted, the trees' flowering time is disrupted and crops can fail, the scientists said.

At the current rate of global warming, the team predicted, the winter chill times in the Central Valley will decrease by 50 percent before the century is up. Without major changes in crop-growing techniques, the Central Valley will no longer be suitable by 2100 for growing such California staples as apples, cherries, pears, peaches, nectarines and walnuts, the scientists said.

Three million acres of orchards are planted in California, yielding crops estimated at $8.7 billion each year, the scientists say. Most of those orchards are in the Central Valley.

"Depending on the pace of winter chill decline, the consequences for California's fruit and nut industries could be devastating," the researchers say in a report published today in the authoritative journal PLoS One, by the Public Library of Science.

Minghua Zhang, professor of environmental and resource science at UC Davis, and Eike Luedeling, a postdoctoral fellow, are the principal authors of the report, along with Evan H. Girvetz, a Davis graduate now at the University of Washington.

California's prune crop is already facing serious problems as each winter's chill time grows briefer, Luedeling said in an interview. "And walnuts are going to have a hard time," he said, because their blooming period is becoming less regular.

"The take-home message," Zhang said in an interview, "is that farmers and consumers need to be prepared for major cultural changes in the tree crops California farmers are growing."

Among those changes, she said, would be new breeding programs based on inducing genetic changes in many of the trees, or finding chemical methods for artificially lengthening dormancy times.

And because new orchards can take years to go from planting to productivity, farmers need to consider looking now for growing areas farther north, Zhang said.